1. Field of the Invention
The present invention relates to a method and system for predicting the print colors of a print produced by a printing press.
2. Description of the Related Art
Prints are produced by generating original film plates in various colors including cyan (C), magenta (M), yellow (Y), and black (K), for example, producing PS plates (presensitized plates) from the original film plates by exposure and development, mounting the PS plates on a printing press such as a rotary press or the like, and adjusting printing conditions, including printing pressure, dampening water, temperature, etc.
Therefore, complex steps are involved in producing prints. In order to produce a print having desired colors, it has been customary, prior to the production of the print, to generate a proof sheet using a simple output device such as a color printer or the like, confirm the generated proof sheet, and adjust printing conditions.
The colors of a print immediately after it has been printed and the colors of the same print upon elapse of a certain period of time after printing are different from each other, because the inks penetrate into the print sheet and are dried over time. This phenomenon is generally referred to as dry-down.
FIG. 8 of the accompanying drawings shows a graph having a horizontal axis representing a time in minutes that has elapsed after a print is produced, and a vertical axis representing a color difference dE from a color upon elapse of a sufficient period of time from the production of the print. It can be seen from FIG. 8 that the color difference dE varies greatly immediately after each of the inks in the colors C, M, Y, K has been printed. The variation of the color difference dE becomes smaller as the time in minutes elapses. The color difference dE also differs from ink to ink.
As shown in FIGS. 9, 10 and 11 of the accompanying drawings, if the print colors are expressed by colorimetric values L*, a*, b*, then on each of an a*b* plane, an L*a* plane, and an L*b* plane, the color gamut of the print, indicated by the dotted-line curve, prior to dry-down immediately after the print is produced is reduced to the color gamut of the print, as indicated by the solid-line curve, subsequent to dry-down. Particularly, the reduction of the color gamut stands out in shadows.
As shown in FIGS. 12, 13 and 14 of the accompanying drawings, if the print colors are expressed by calorimetric values X, Y, Z in graphs having horizontal axes representing a halftone dot percentage (%) and vertical axes representing the colorimetric values X, Y, Z, then the colorimetric values X, Y, Z change, so as to become increased from the values indicated by the dotted-line curve prior to dry-down to the values indicated by the solid-line curve after dry-down. The changes in the colorimetric values X, Y, Z differ depending on the halftone dot percentage (%).
In actual printing environments, it is necessary to confirm the color tones of proof sheets in view of such a dry-down phenomenon. Since it takes a long period of time for the color tones of a proof sheet to become stabilized, it has been customary for operators in the printing industry to compare a printed sample, which corresponds to a print subsequent to dry-down (hereinafter also referred to as a post-dry-down print), and a post-dry-down print, which is estimated based on experience from a print prior to dry-down (hereinafter also referred to as a pre-dry-down print). Accordingly, the adjustment of colors tends to differ depending on the experience of the operator.
According to a process of adjusting colors based on a pre-dry-down print, as disclosed in Japanese Laid-Open Patent Publication No. 2004-230793, the pre-dry-down color tones of respective colors formed on a print sheet are measured, changes caused in the post-dry-down color tones are stored in advance, and the pre-dry-down color tones are compared with pre-dry-down target values, which are represented by the sum of the changes and reference values of the post-dry-down color tones.
Japanese Laid-Open Patent Publication No. 2004-251848 discloses a color tone manager for generating a pre-dry-down profile and a post-dry-down profile, which represents a color conversion relationship from colorimetric values of a pre-dry-down color chart and colorimetric values of the post-dry-down color chart, and then simulating pre-dry-down and post-dry-down color tones using the generated profiles.
According to the above schemes of the related art, it is necessary to print a color chart of many patches in order to obtain changes in the pre-dry-down and post-dry-down color tones, or to obtain pre-dry-down calorimetric values. A color chart made up of combinations of four colors C, M, Y, K, for example, needs to have about one thousand patches in order to produce highly accurate color adjustments. Since a considerable period of time is required to calorimetrically measure so many patches, the colors of the patches tend to vary during measurement, as shown in FIG. 8. As a result, the obtained colorimetric values are of low reliability.